Plastic substrates, such as polycarbonates, are used in a wide variety of applications. Unfortunately, such substrates are often particularly susceptible to environmental damage, such as scratching, as a result of impact with projectiles and other articles. As a result, to protect the substrate from such damage, the substrate may be coated with a clear and optically transparent “self-healing” coating that comprises a flexible polyurethane material. These coatings are often described as “self-healing” coatings because of their ability to return to their original shape or appearance after being deformed.
Generally speaking, polyurethanes are produced by reacting an isocyanate, a compound with functional groups of the type —NCO, with a hydroxyl, —OH, functional material. To produce storage stable one component compositions that form a polyurethane, blocking agents for temporary protection of isocyanate groups are often used. The reversible blocking of the —NCO groups allows the preparation of one-pack compositions containing a blocked polyisocyanate and, for example, a polyol, which can be cured to form a polyurethane by, for example, thermal treatment.
As will be appreciated, however, during thermal cure, the blocking agent is released and may, to a certain extent, remain in the coating. Historically, polycarbonate substrates have been susceptible to chemical attack by these blocking agents, particularly when the coating is applied film thicknesses of at least 1 mil (25.4 microns), in some cases 1 to 5 (25.4 to 127 microns) or 2 to 5 mils (50.8 to 127 microns), which are often required to form a “self-healing” coating. This chemical attack causes a haze in the resulting coating. In addition, as will be appreciated, plastic substrates, such as polycarbonates, are not able to withstand extremely high temperatures. As a result, when coating such substrates, it is important that the blocking agent deblock at a relatively low temperature.
As a result, it is desirable to provide methods for making plastic substrates, such as polycarbonate substrates, at least partially coated with a haze-free, self-healing coating.